At present, no industrially usable process for the underwater drilling a superthick metal plate in excess of 150 mm has yet been developed, and particularly, it is difficult to make a small hole in such a plate having a great wall thickness.
Even a superthick plate can be drilled using a special lengthy drill if a long period of time is taken, but this involves an extremely expensive operation.
In this case, the drill has to be replaced frequently in the course of drilling, and if the drill should break, it is extremely difficult to remove the drill from the hole.
Such drilling of superthick plate is required only in special fields but in these fields is an indispensable process. For example, in the case gas cutting is employed for the underwater disassembly of the pressure vessel of a discarded nuclear reactor, since the pressure vessel is formed from superthick plates, gas cutting cannot be used unless edges or through holes are present.
This is because the principle of gas cutting requires that the matrix be preheated to about 1,000.degree. C. before a flow of cutting oxygen is projected thereonto to utilize the oxidization reaction between iron and the oxygen. However, in the case of a superthick plate, heat is absorbed into the matrix by heat transfer unless an end (edge) or hole edge is present in the plate, and therefore it is hard to apply heat thereto. Even if heating can somehow be realized, when the cutting flow of oxygen is blown to carry out cutting, the cutting chips or molten metal (dross) fly up into the blow-pipe nozzle since there is no place for the chips and dross to escape. Therefore, the nozzle becomes damaged at once. However, if the plate has ends or holes, the cutting chips or molten metal (hereinafter referred as "molten metal") may be readily released to the rear and therefore, cutting can be accomplished.
Since a cylindrical vessel such as the pressure vessel of a reactor has no edges, drilling of start-holes is an indispensable step for application of gas cutting.
If a mechanical process using drills or the like for drilling start-holes to permit use of gas cutting for underwater disassembly of the pressure vessel of a discarded reactor is used as described above, the following problems arise in addition to the aforementioned disadvantages. Namely, the operation takes a long time and the drills have to be replaced frequently. This is disadvantageous in terms of safety because the operators are required frequently and for a relatively long times, to be in contact with tools and parts wet with radioactive water. Furthermore, in the mechanical drilling of the superthick plate, a great pressure must be applied, and therefore, disadvantageously, a large-scale apparatus must be used.